1. Field of the Invention
The invention pertains to the field of devices for tensioning flexible drive transmission means, such as belts or chains. More particularly, the invention pertains to hydraulic tensioners. Reference will be made to transmission chains for internal combustion engines, such as are used to transmit motion from a first driving sprocket, operated directly or not by a vehicle engine, to one or more driven shafts, for example a cam shaft, fuel injection pump, or oil pump.
2. Description of Related Art
For reasons of adjustment, wear on materials and take-up of slack, it is often necessary to compensate for a certain slack of the chain and this is done by means of shoe tensioners, in which a shoe is biased with adjustable force against a chain side.
The bias on the shoe is normally obtained by a hydraulic tensioner. In its most widely known embodiment, a hydraulic tensioner comprises a cylinder-piston assembly. A stationary member (generally the cylinder) is mounted on the engine block, and a movable member (generally the piston) acts on the shoe under the action of a spring and hydraulic fluid, generally oil. In some known tensioners the piston is hollow and the piston chamber or bore receives a vent device sliding axially therein. The vent device has an end part that can be hemispherical or flat with a thin spiral groove on the face thereof facing toward the bottom of the piston, and the bottom of the piston has a through hole. The device is biased by the spring against the bottom of the piston and allows any air to exit or possibly oil to be discharged for lubrication purposes and to adjust the elastic-damping characteristics of the tensioner. The spring therefore acts between the cylinder and the disk and, through it, acts on the piston.
Some tensioners include a so-called “no-return” device, able to prevent re-entry of the piston into the cylinder if the oil pressure is lost and the action of the spring is not sufficient to maintain the piston in the correct position. The no-return device generally consists of a rack or toothed portion integral with the piston or in any case with the mobile member, which is engaged by a spring-biased toothed or ratchet locking pad.
Tensioners that can be fitted from the outside in the engine block or head, without any need to open the engine are called “cartridge” tensioners. EP-A-00830616.9, not yet published, describes a cartridge tensioner in which the rack portion is formed on the piston skirt. The ratchet pad is received in an opening in the cylinder skirt, with the possibility of axial sliding, and biased against the piston by a circular elastic band. For storage and shipping, before installation the piston is retained in a retracted position, in which only a small portion thereof extends from the cylinder, by engagement of a snap ring or circlip with an inside annular recess of the cylinder and an outside annular recess of the piston. To install the tensioner, it is necessary first to apply a force to the piston head which retracts it inside the cylinder for a length sufficient to move the retaining snap ring into a second, wider internal annular recess of the cylinder, so that the ring releases the piston. Although the solution provided in the aforementioned application is satisfactory for many applications, it nevertheless is not suitable in those situations where it is not possible or easy to apply the retraction force to the piston head.
U.S. Pat. No. 5,700,215 discloses a hydraulic tensioner having a cylinder and a solid piston or plunger movable inside the cylinder, in which a stopper screw for the piston is rotatably received in a through aperture of the cylinder wall and an internally threaded recess is formed on the inner end face of the piston. The screw, when engaged in the recess of the piston, retains the piston in a retracted condition, as can be required, for example, for maintenance purposes. This patent does not solve the problem concerning hollow piston or cartridge tensioners.